The object of the invention is to carry out a steam-cracking of light feedstocks under very severe conditions and at a very high conversion level, in particular for feedstocks that are high in ethane or in a C2/C3 mixture.
Typically, these feedstocks contain at least 20% by weight of ethane and/or propane and at least 80% by weight of hydrocarbons with 2 and/or 3 carbon atoms. Most generally, these are mixtures of ethane and propane, which can also comprise variable amounts of propylene, as well as small amounts of ethylene, methane and hydrocarbons with 4 carbon atoms or more.
The steam-cracking of light feedstocks is a process that is extensively described in technical literature, for example in the work that is well known in the steam-cracking industry: xe2x80x9cETHYLENE KEYSTONE TO THE PETROCHEMICAL INDUSTRYxe2x80x9d Ludwig Kniel, Olaf Winter, Karl Stork, Editor MARCEL DEKKER, INC., New York, 1980, (Reference 1).
The technological background is described in, for example, patents U.S. Pat. No. 4,762,958, FR-A 2 032 437 and FR-A 2 760 465.
Ethane, and to a lesser extent propane, is a feedstock that is fairly refractory for which it is difficult to obtain high conversion levels. The high conversion levels require very rigorous operating conditions of furnaces that result in high coking speeds that increase the skin temperatures of pyrolysis tubes and reduce the cycle times. The work that is cited provides ethane steam-cracking yields, page 65, at 50% and 60% of conversion, value typically used in steam-cracking furnaces. It is indicated on page 112 of this work that a conversion of 70% is beyond industrial possibilities with the technologies that are known at this time. The cracking furnaces actually comprise tubes or coils for circulating a gas mixture that contains water vapor, ethane and products that are obtained from cracking. These tubes are made of refractory alloys that are high in chromium, nickel and iron, such as the xe2x80x9cHK40xe2x80x9d alloy that is well known to one skilled in the art and that consists mainly of 25% chromium, 20% nickel and a balance of iron (aside from minor additions).
These alloys are limited in their operating conditions by metallurgic constraints, which limits the conversion of ethane and/or propane.
This does not mean that it is impossible for a given furnace to exceed nominal conversion, for example 60%, but if the heating of the burners is advanced to increase this conversion, it will not be possible to avoid an accelerated aging of the tubes and ruptures of the tubes, in particular by creep and carburation. The cycle times are also reduced very significantly because of a drastic increase of coking under very severe conditions required for high conversion levels of ethane and/or propane.
The manufacturers of pyrolysis tubes have made significant progress, however, and have developed refractory alloys that are higher-performing than the HK40: In particular the 25/35 alloys (Cr, Ni) with a balance of Fe plus some additions (Si, Mn, Nb, Ti . . . ), for example the xe2x80x9cHP MODxe2x80x9d alloys, are known. More recently, 35/45 alloys (Cr, Ni), and even alloys with 40 or 45% of chromium, which contain less than 15% of iron, with a balance of nickel, aside from minor additions including 1 to 2.5% of silicon, were developed.
A list of materials for pyrolysis tubes, with their recommended use-limit compositions and temperatures is provided in xe2x80x9cProceedings of the 10th Ethylene Producers Conferencexe2x80x9d (1998) published by AlChE (Reference 2) in the article xe2x80x9cCoke Reduction and Coil Life Extension,xe2x80x9d pp. 107-108 (Reference 2A).
With the highest-performing materials, the practicable conversion of ethane industrially has been brought to about 65%.
The thermal fluxes that are used in the cracking zone with radiation are variable according to the types of furnaces but between 50 and 120 KW/m2, if they are related to the outside surface of the tubes. It is possible in particular to refer to the work of Reference 1, page 131, which mentions fluxes between 50 and 80 KW/m2. It is also possible to refer to the work: xe2x80x9cPROCEDES DE PETROCHIMIE [PROCESSES OF PETROCHEMISTRY],xe2x80x9d Vol. 1, TECHNIP Editions, Paris, 1985 of A. Chauvel, G. Lefebvre, L. Castex (Reference 3), page 159, which mentions the mean fluxes between 75.5 and 104.5 KW/m2.
Flow values of 50 KW correspond to the use of old HK40-type alloys, which are no longer used for the hottest end portion of the pyrolysis coils. With modern alloys, the fluxes that are used are typically between 80 and 120 KW. A correspondence between the flux and the maximum skin temperatures of the tubes is given in Reference 3, page 160.
Obviously, the tendency of the steam-cracking industry is to increase the thermal fluxes, by taking advantage of the better alloys that are available, to increase productivity as well as the level of severity of the steam-cracking (i.e., the conversion in the case of ethane).
Whereby the advances of the alloys are not without limit, it is difficult today, however, to exceed approximately 65% of conversion for ethane.
The metallurgists have therefore turned, in the most recent state of the art, to new improvements, in other directions:
Thus, two publications:
xe2x80x9cA Low Coking Environment for Pyrolysis Furnacesxe2x80x94CoatAlloyxe2x80x94, M. Bergeron, E. Makarajh, T. McCall,xe2x80x9d and xe2x80x9cResults of a Furnace Tube Surface Treatment in a Full Furnace Trial, D. Mullenix, A. Kurlekarxe2x80x9d
were presented in the conference: xe2x80x9c1999, AlChE Spring National Meeting, Eleven Annual Ethylene Producers Conference Mar. 16, 1999 HOUSTON, Tex.xe2x80x9d (Reference 4).
In these publications, recent (1998) industrial results of ethane steam-cracking with pyrolysis tubes comprising anti-coking surface coatings are presented. According to these publications, the limitation of coking makes it possible to increase the thermal fluxes and the conversion and to operate in a satisfactory manner (cycle time, behavior of materials) at a conversion of 70%. It was also proposed to use tubes that comprise a welded inside fin, in particular a helicoidal one, which has the effect of increasing the thermal transfer, and therefore the performances of the furnaces that use this technique.
To exceed this maximum conversion limit, developments have also been undertaken to use ceramic pyrolysis tubes or pipes. The operating temperatures of the ceramics are extremely high, and these materials completely eliminate the catalytic coking. A plan for a furnace with ceramic pyrolysis tubes for high-conversion ethane cracking was thus presented by one of the major engineering firms that build steam-cracking devices (Reference 2, article xe2x80x9cCoke-free Crackingxe2x80x94Is It Possiblexe2x80x9d by Khoi (Paul) X Pham, Dennis Duncan, Joseph M. Gondolfe (Reference 2B), pp. 127-150).
In this article, it is shown that cycle times of at least 7 days were obtained with a ceramic tube for an ethane conversion of 75% (p. 139). It is indicated that the metallic tube of the same geometry led to clogging within 3 hours. The dwell times used are very short (less than 50 milliseconds (ms)), which is in accordance with the philosophy and the evolution of the steam-cracking industry for several decades.
The ethane conversions at 77% and more thus are not currently being envisioned and considered as possible within the scope of an industrial operation except with very particular and complex furnace designs that use ceramic materials.
The orientations of the steam-cracking industry of light feedstocks for the preferred production of ethylene are thus:
Use of the highest-performing refractory alloys and, moreover, ceramic materials.
Use of the highest acceptable thermal fluxes.
Use of the shortest possible dwell times (in particular less than or equal to 100 milliseconds).
The invention has as its object a steam-cracking process that makes it possible to crack the ethane and light feedstocks with conversions that are greater than or equal to 77% and even 80% or even greater than 95%. This process makes it possible not only to be able to carry out these conversions on a pilot laboratory furnace or on an industrial furnace on a temporary basis, but on industrial furnaces with operating conditions that are compatible with the requirements of the industry (service life of the tubes, cycle times, etc. . . . ).
Surprisingly and contrary to prior teaching, it was discovered that it is possible to obtain very high conversion levels mentioned above by using a steam-cracking process under very severe conditions of a feedstock that comprises at least 80% by weight of hydrocarbons that have 2 to 4 carbon atoms, and at least 20% by weight of hydrocarbons of the group of ethane and propane that is characterized in that:
The feedstock that is diluted with water vapor in a radiation zone of a furnace is circulated in at least one pipe with length Lxe2x89xa716 m and a hydraulic diameter that is greater than or equal to 34 mm in the end portion of the pipe at least, under the following conditions of dwell time xcfx84 and furnace output temperature COT:
150 msxe2x89xa6xcfx84xe2x89xa62800 ms;
858xc2x0 C.xe2x89xa6COTxe2x89xa61025xc2x0 C.,
whereby the ratio of L/mean hydraulic diameter DH, and dwell time xcfx84 is large enough to obtain at least one of the following results:
A conversion of at least 77% of the ethane of the feedstock, if the feedstock contains ethane.
A conversion of at least 96% of the propane of the feedstock, if the feedstock contains propane,
whereby thermal flux xc3x8 in the radiation zone is low enough so that parameter "xgr"1 that is defined by:   ξ1  =                    [                  xe2x88x85          *                ]            2        R  
with:
R=L/DH, xc3x8*=xc3x8xc3x97F
where xc3x8 is the mean thermal flux that is sent into the pipe in the radiation zone, in KW per m2 of outside surface of the pipe, and F=
0.85 if the end portion of the pipe comprises at least one inside fin or an anti-coking coating on at least one portion of its inside surface,
0.72 if the end portion of the pipe comprises at least one inside fin and an anti-coking coating on at least one portion of its inside surface,
1 otherwise,
confirms "xgr"1xe2x89xa611, whereby the value of "xgr"1 is low enough to maintain this conversion for a cycle time that is greater than or equal to about 8 days, in particular at least 10 days, in particular at least 15 days, more particularly at least 20 days and typically at least 30 days.
Conventionally, for the definition of dwell time xcfx84, only a portion of the radiation zone in which the feedstock reached or exceeded the conventional temperature at the very beginning of steam-cracking of 650xc2x0 C. is considered. The retention time is therefore conventionally the dwell time between the point where the temperature reaches 650xc2x0 C. (or the input of the radiation zone if the input temperature is greater than 650xc2x0 C.) and the starting point of quenching of cracked gases at the output of the radiation zone of the furnace. This dwell time therefore comprises the dwell time in the adiabatic transfer line that goes from the output of the radiation zone of the furnace to a quenching exchanger and, if necessary, the dwell time in the input cone of this exchanger.
The steam-cracking process according to this invention can preferably be applied in the case where the feedstock comprises at least 20% by weight of ethane, whereby the conversion of the ethane of the feedstock is greater than or equal to 77%, or a conversion CONVxe2x89xa777.
If the feedstock does not consist of pure ethane, the residual ethane does not reflect exactly the unconverted fraction of the ethane of the feedstock. Actually, when a feedstock that also comprises other hydrocarbons is cracked, these hydrocarbons provide during their cracking significant amounts of ethane which are added to the ethane initially contained in the feedstock. Thus, if the residual ethane represents 20% or 13% of the initial ethane, the initial ethane conversion was greater than 80% (or 87%).
Typically, for the implementation of this invention, a tubular steam-cracking furnace that comprises at least one pipe or coil in the radiation zone whose materials are basically metal alloys is used. The boundary temperature of the materials of the end portion of the pipe is preferably greater than or equal to 1060xc2x0 C., in particular 1100xc2x0 C., and in particular greater than or equal to 1120xc2x0 C., for example between 1120 and 1220xc2x0 C.
Conventionally, the end portion of the pipe is the downstream portion of the pipe that corresponds to 25% of total length L. Conventionally, boundary temperature TL of a pyrolysis tube material is the maximum temperature that can be reached by this tube (skin temperature) at the end of the cycle before triggering a decoking procedure. This maximum acceptable temperature is therefore dependent, within a limited range, on the selection of the manufacturer that operates the steam-cracking device, which can use a relatively low value of TL to favor the service life of the tubes or a higher value to increase the cycle time or the conversion. The recommended values are indicated in Reference 2.
A process is proposed in particular in which:
20xe2x89xa6xc3x8*xe2x89xa679, in particular 25xe2x89xa6xc3x8*xe2x89xa670, more particularly 25xe2x89xa6xc3x8*xe2x89xa664, in particular 28xe2x89xa6xc3x8*xe2x89xa659, and, for example, 30xe2x89xa6xc3x8*xe2x89xa655.
A pipe or steam-cracking coil typically comprises a number (2, 3, 4, 5 or more) of sections with a different inside diameter that generally increases when going from the upstream direction to the downstream direction. It can also have clusterings of tubes between one another. Conventionally, the ratio of   R  =      L          D      H      
is calculated by adding (by going from the upstream direction to the downstream direction) of the length to inside diameter ratios of each section. If a coil comprises several tubes that operate in parallel with their clustering, it is necessary to take into consideration only one of them, by following a single flow line. By definition, there is then       D    H    =            L      R        .  
The ratio of   R  =      L          D      H      
is generally greater than or equal to 560 and less than or equal to 1400, preferably confirming 1400xe2x89xa7Rxe2x89xa7620, and typically such that 1300xe2x89xa7Rxe2x89xa7670, in particular 1200xe2x89xa7Rxe2x89xa7700, in particular 1150xe2x89xa7Rxe2x89xa7740, and for example 1100xe2x89xa7Rxe2x89xa7770.
According to the invention, high ratios of pipe length to pipe diameter and low fluxes are usually used. xc3x8* is a corrected flux to take into account the possible usage either of anti-coking surface deposits or tubes that comprise straight or helicoidal fin that improve the thermal transfer, whereby these techniques make it possible to increase the usable thermal flux. Design parameter "xgr"1 takes into account both the ratio R=L/DH and corrected flux xc3x8*.
Without being linked by any theory, it was found that the current limitations of the ethane conversion resulted from two phenomena:
1) Kinetic Aspect:
Let us consider, for example, a recent furnace design that from a conventional point of 650xc2x0 C. uses a thermal flux of 90 KW/m2, a length of coil L that makes it possible to obtain at the furnace outlet a conversion of 65%. If it is desired to increase the conversion tc reach 80%, notably the thermal flux will logically be increased beyond 90 KW/m2. This will have the result of increasing the conversion, but also the reaction temperatures. In any steam-cracking furnace, there is competition between the heat input, which tends to increase the temperature of the reaction medium, and the kinetics of cracking which tends to cool the reaction medium because of the endothermic reactions of cracking. When it is desired to achieve very high conversion levels such as 77 to 97%, the thermokinetic modelization of the pyrolysis coil makes it possible to analyze that at the end of the reaction zone, a negative factor opposing the cooling of the medium is the significant reduction of the hydrocarbons and the ethane that are not yet cracked. The result is that the output temperature greatly increases without the cooling by cracking being adequate to moderate this temperature increase. This phenomenon is all the more significant as the thermal flux is high. The result is a significant increase of the temperatures of the furnace output and the tube skin, with an accelerated coking, which means that it cannot work on an industrial scale.
If, in contrast and paradoxically, low thermal fluxes are used, lower than those currently employed under industrial conditions, compensated by an increase of the L/DH ratio, it is possible to reestablish a favorable balance in maintaining acceptable temperatures in the competition between heating and cooling: time is allowed for the reaction to continue, and the skin temperatures remain moderate, which makes it possible to preserve an acceptable coking speed.
Preferably "xgr"1 is such that 1xe2x89xa6"xgr"1xe2x89xa69.5, for example 1xe2x89xa6"xgr"1xe2x89xa68.5, in particular 1.5xe2x89xa6"xgr"1xe2x89xa67, in particular 1xe2x89xa6"xgr"1xe2x89xa66; the recommended values of "xgr"1 are such that 1.3xe2x89xa6"xgr"1xe2x89xa65.2 and particularly 1.4xe2x89xa6"xgr"1xe2x89xa64.5.
A process is also presented in which parameter "xgr"2 is defined by:       ξ    2    =                              [                      xe2x88x85            *                    ]                2            R        xc3x97          f      ⁡              (        CONV        )              xc3x97          g      ⁡              (                  T          MT                )            
with       f    ⁢          (      CONV      )        =      1    +          0.4      ⁢                                    CONV            -            77                    100                    
where CONV is the conversion percentage of the ethane of the feedstock,
and       g    ⁢          (              T        MT            )        =      5.07    xc3x97          10      9        ⁢          ⅇ              (                              -            28000                                              T              MT                        +            273                          )            
with TMT (xc2x0 C.)=maximum temperature of the skin of the tubes at the beginning of the cycle in the portion of the radiation zone that corresponds to a dwell time that is greater than or equal to 110 ms,
is such that: 1xe2x89xa6"xgr"2xe2x89xa620, in particular 1xe2x89xa6"xgr"2xe2x89xa615, in particular 1xe2x89xa6"xgr"2xe2x89xa613, in particular 1xe2x89xa6"xgr"2xe2x89xa611 and preferably 1xe2x89xa6"xgr"2xe2x89xa69, whereby flux xc3x8 is adequate, but "xgr"2 is low enough to reach a conversion percentage of the ethane of the feedstock: CONV greater than 77 and to maintain it (to be able to maintain it) at this level for a cycle time of at least 8 days, in particular at least 10 days, in particular at least 15 days, more particularly at least 20 days and typically at least 30 days.
"xgr" is the parameter of modified design, to take into account the level of conversion, on the one hand, and the maximum temperature of the skin of the tubes on the other hand (observed beyond the initial cracking zone).
A process is also presented in which ratio R is selected, and thermal flux xc3x8* is adjusted such that parameter "xgr"3 that is defined by       ξ    3    =            Δ      ⁢              xe2x80x83            ⁢              T        OPER                            xe2x88x85        *            xc3x97              f        ⁡                  (          CONV          )                    xc3x97              g        ⁢                  (                      T            MT                    )                    
is such as
5xe2x89xa7"xgr"3xe2x89xa70.25, and in particular
4xe2x89xa7"xgr"3xe2x89xa70.5, in particular 4xe2x89xa7"xgr"3xe2x89xa70.6, more particularly
4xe2x89xa7"xgr"3xe2x89xa70.7, and preferably 3.5xe2x89xa7"xgr"3xe2x89xa70.8,
with xcex94TOPER=TLxe2x88x92TMTxe2x89xa730xc2x0 C., where TL is the boundary operating temperature of the end portion of the pipe results, whereby flux xc3x8 is adequate, but "xgr"3 is large enough to reach a conversion percentage CONV greater than 77 and to maintain it (to be able to maintain it) at this level for at least one cycle time of at least 8 days, in particular at least 10 days, in particular at least 15 days, more particularly at least 20 days and typically at least 30 days.
"xgr"3 is the operating parameter, whose evolution mainly reflects the cycle time of the furnace. The larger "xgr"3 is, the higher the cycle time. The expression of "xgr"3 shows that it is necessary to have a boundary temperature TL that is significantly greater than the maximum skin temperature at the beginning of the cycle (to allow the coking to take place and to increase the skin temperature during a high cycle time). It is also necessary to have a relatively low skin temperature TMT so that correcting coefficient g(TMT) is the smallest possible (whereby this coefficient reflects a coking speed index, linked to the skin temperature).
Length L of the pipe can be defined by the equation:   L  ≥      K    xc3x97                  Q        xc3x97        Δ        ⁢                  xe2x80x83                ⁢                  H          70                            xe2x88x85        ⁢                  xe2x80x83                ⁢                  S          L                      ⁢          xe2x80x83        ⁢    with  xe2x80x832xe2x89xa7Kxe2x89xa71.05, and in particular
1.6xe2x89xa7Kxe2x89xa71.1
where:
Q is the mass flow rate of the feedstock that is diluted with water vapor, Kg/s, xcex94H70 is the transferred heat that corresponds to a conversion of 70% of the ethane, Kj/Kg, SL is the mean linear outside surface of the pipe in the heating zone: m2/m.
Dwell time xcfx84 is advantageously such that:
250 msxe2x89xa6xcfx84xe2x89xa61500 ms, more particularly 350 msxe2x89xa6xcfx84xe2x89xa61200 ms, in particular 350 msxe2x89xa7xcfx84xe2x89xa7100 ms, and typically 400 msxe2x89xa6xcfx84xe2x89xa6950 ms.
If ppHC (bar absolute) is used to represent the partial pressure of hydrocarbons+hydrogen in the effluents, in furnace output, then generally: 0.2xe2x89xa6ppHCxe2x89xa62.8, in particular 0.3xe2x89xa6ppHCxe2x89xa62.8 and preferably 0.4xe2x89xa6ppHCxe2x89xa61.6.
The furnace cutput temperature is typically such that: COTxe2x89xa7876xc2x0 C., and in particular confirms
980xe2x89xa7COTxe2x89xa7858+10 [f(CONV)+ppHC]
and COTxe2x89xa7876xc2x0 C.
These values make it possible not to come too close to the values of the ethane/ethylene equilibrium (whereby COT is greater than the equilibrium temperature). When work is done at a very high conversion level, for example CONVxe2x89xa790, it is preferable to use COT temperatures xe2x89xa7910xc2x0 C.
According to a variant of the process, the effluents of the radiation zone can pass through an approximately adiabatic zone with a dwell time xcfx84* such that: 1 msxe2x89xa6xcfx84*xe2x89xa6220 ms, and in particular 8 msxe2x89xa6xcfx84*xe2x89xa680 ms, before their input into a quenching zone (ms=milliseconds).
The maximum temperature of the tubes"" skin at the beginning of the cycle is generally kept at a moderate value, generally: 975xc2x0 C.xe2x89xa6TMTxe2x89xa61060xc2x0 C, and in particular 980xc2x0 C.xe2x89xa6TMTxe2x89xa61050xc2x0 C. and in particular 990xc2x0 C.xe2x89xa6TMTxe2x89xa61044xc2x0 C., and preferably 1000xc2x0 C.xe2x89xa6TMTxe2x89xa61038xc2x0 C.
The sizing parameters of the furnace will preferably be selected so as to have:
50xc2x0 C.xe2x89xa6TMTxe2x88x92COTxe2x89xa6130xc2x0 C., and in particular:
80xc2x0 C.xe2x89xa6TMTxe2x88x92COTxe2x89xa6106xc2x0 C.
According to a characteristic variant of the process, the end portion of the pipe can comprise at least one tubular element of the group that is formed by the pyrolysis tubes with an anti-coking inside coating and pyrolysis tubes that comprise an approximately helicoidal inside fin. This makes it possible to lower the maximum skin temperature TMT and to increase the cycle time.
Typically, the feedstock can circulate in pyrolysis tubes whose inside diameters are between:
45 and 180 mm, in particular between 65 and 130 mm.
Typically, dilution rate d of the feedstock by the water vapor (fraction by weight of water vapor/hydrocarbon feedstock) is such that 0.05xe2x89xa6dxe2x89xa62, in particular 0.15xe2x89xa6dxe2x89xa61 and preferably 0.2xe2x89xa6dxe2x89xa60.8.
The cycle time is typically at least 15 days, in particular at least 20 days and preferably greater than or equal to 30 days.
The conversion of ethane is generally selected so that the residual ethane that is contained in the effluents of the quenching zone is less than or equal to 24% by weight, in particular 15% by weight, of the ethane that is contained in the feedstock. Typically, the conversion of the ethane of the feedstock is greater than or equal to 85%, in particular CONVxe2x89xa788, in particular CONVxe2x89xa790. If the feedstock contains propane, a conversion of the propane of the feedstock that is greater than or equal to 97%, in particular 98%, will preferably be selected.
The invention also proposes, for the execution of the process, a steam-cracking furnace under very severe conditions that comprises a convection zone for the preheating of a hydrocarbon feedstock that comprises at least 20% by weight of hydrocarbons with 2 or 3 carbon atoms, diluted with water vapor, a radiation zone that is connected upstream from the convection zone and downstream from a quenching zone, whereby the radiation zone comprises a number of pipes of ratio   L      D    H  
that is greater than or equal to 680 and heating means of the feedstock that is diluted with water vapor at a temperature of at least 876xc2x0 C.
Typically, it is possible to have:       2000    ≥          L              D        H              ≥    700    ,
in particular:   2000  ≥      L          D      H        ≥  760.
The furnace comprises in particular heating means that deliver to said pipes a nominal flux xc3x8 that confirms 20 KW/m2xe2x89xa6xc3x8xe2x89xa679 KW/m2, and in particular 25 KW/m2xe2x89xa6xc3x8xe2x89xa664 KW/m2.
The end portion of the pipes typically has a boundary operating temperature TLxe2x89xa71100xc2x0 C., in particular 1220xc2x0 C.xe2x89xa7TLxe2x89xa71120xc2x0 C.